1. Field of the Invention
The present invention relates to a variable directional microphone unit in which a physical structure thereof and a structure of a circuit for electrical switching used therewith can be made simple, and to a variable directional microphone.
2. Description of the Related Art
As a microphone with variable directionality, a microphone is known that has a microphone unit composed of two capacitor microphone units connected back-to-back (see, for example, Japanese Patent Application Laid-open No. H7-143595 and No. 2008-67286). Both microphone units have cardioid characteristic. The variable directionality is achieved through adjusting their outputs or, as described in Japanese Patent Application Laid-open No. H7-143595, through adjusting polarization voltages applied to each element.
An example of a conventional variable directional microphone unit similar to those of Japanese Patent Application Laid-open No. H7-143595 and No. 2008-67286 is shown in FIG. 11. In FIGS. 11 to 14, a variable directional capacitor microphone unit is composed of two individually formed capacitor microphone units 21 and 41 connected back-to-back. A diaphragm-like vibrating plate 22 has its outer peripheral portion fixed to one side of a vibrating plate holding ring 23 to compose a vibrating plate assembly therewith. The vibrating plate holding ring 23 is made of a conductive material and an electrode plate 24 having a plurality of acoustic terminal holes 241 is disposed thereon. An electrode 25 electrically conducted to the vibrating plate 22 is fixed to the electrode plate 24. The vibrating plate 22 integrally held by the vibrating plate holding ring 23 is placed on a disk-shaped fixed electrode 26 with a ring-shaped spacer 27 made of an extremely thin insulating material in between. Thus, the vibrating plate 22 faces an upper surface of the fixed electrode 26 with a slight gap with a size corresponding to a thickness of the spacer 27 in between. The spacer 27 is sandwiched by the vibrating plate 22 and the fixed electrode 26 at the position near their outer peripheries.
The fixed electrode 26 is placed on an insulative base 30 with a receiving ring 28 in between. The base 30 has circular flanges 31 and 32 formed along peripheries of an upper and a lower surface thereof, respectively. The receiving ring 28 and the fixed electrode 26 are dropped into a space surrounded by the flange 31. Both upper and lower surfaces of the base 30 gradually inclined towards the center and a vertically through hole is formed at the center. An acoustic resisting member 34 is fit into the hole. A upper surface of the fixed electrode 26 protrudes above that of the flange 31. The spacer 27, the vibrating plate assembly formed of the vibrating plate holding ring 23 and the vibrating plate 22, and the electrode plate 24 are stacked on the fixed electrode 26 in this order. A holding ring 29 is fit around the outer peripheries of the electrode plate 24 and the vibrating plate assembly. The holding ring 29 is also fit around an outer periphery of the flange 31 of the base 30 to be fixed thereto with any appropriate fixing methods. An upper edge of the holding ring 29 is formed to be an inner extending edge 291. As the inner extending edge 291 pushes down the electrode plate 24, the units described above are secured to the base 30 by being urged thereto. The microphone unit 21 is thus formed.
The acoustic terminal holes 241 of the electrode plate 24 serve as a front acoustic terminal of the microphone unit 21. The fixed electrode 26 has a plurality of holes as well. Through the acoustic terminal holes 241 of the electrode plate 24 and the holes formed on the fixed electrode 26, a space behind the vibrating plate 22 is communicated with: a space formed by the upper surface of the base 30 being gradually inclined towards the center; and, via the acoustic resisting member 34, a space formed by the lower surface of the base 30 being gradually inclined towards the center.
The other microphone unit 41 has a similar structure as the above described microphone unit 21 connected back-to-back therewith. The microphone unit 41 includes: a vibrating plate 42; a vibrating plate holding ring 43; an electrode plate 44; an electrode 45; a fixed electrode 46; a spacer 47; a receiving ring 48; a holding ring 49; a front acoustic terminal 441 formed of a plurality of holes; and an inner extending edge 491 of the holding ring 49. Above elements have similar structures as that of the corresponding elements of the microphone unit 21. At the lower surface side of the base 30, the microphone unit 41 is formed as the counterpart of the microphone unit 21. Polarization voltages are individually applied to the vibrating plates 22 and 42 of the microphone units 21 and 41.
FIG. 15 depicts an equivalent circuit of the above described microphone unit. In the figure, the two microphone units are connected to each other via acoustic resistance r1 of the acoustic resisting member 34. The microphone unit 21 is at the left of the acoustic resistance r1 while the microphone unit 41 is at the right thereof. In the figure, the microphone unit 21 includes: sound pressure P1; mass mOA, stiffness sOA, and acoustic resistance rOA of a front air chamber; and stiffness S1A of the hole formed in the fixed electrode and a rear air chamber in communication therewith. Similarly, the microphone unit 41 includes: sound pressure P2; mass mOB, stiffness sOB, and acoustic resistance rOB of a front air chamber; and stiffness S1B of the hole formed in the fixed electrode and a rear air chamber in communication therewith.
FIG. 16 depicts an example of a directionality switching circuit that can be applied to the conventional variable directional microphone unit. Constant polarization voltage is applied to the vibrating plate of one of the microphone units 21 and 41 and a level of polarization voltage applied to the vibrating plate of the other microphone unit is switched. Thus, the directionality of the microphone unit can be switched. The exemplary circuit shown in FIG. 16 has DC power sources of +60V and −60V, and +60V is constantly applied to the vibrating plate of one of the microphone units. Voltages of both power sources of +60V and −60V are divided into two levels (for example, into +60V and +30V, and −60V and −30V). Thus, five levels (including 0V) of voltages are generated. The voltage to be applied to the vibrating plate of the other microphone unit is selected from the five levels by means of a switch. 0V (no voltage) is applied to the fixed electrode (also referred to as “a back plate”) included in both microphone units.
FIG. 17 depicts examples of directionalities of the microphone unit obtained through switching between the polarization voltages of different levels by means of the switch. Under a condition in which a contact 1 is selected with a switch shown in FIG. 16, the polarization voltage of +60V is applied to one of the microphone unit whereas the polarization voltage of −60V is applied to the other microphone unit. Here, the microphone unit has bidirectional characteristic as shown in “1” of FIG. 17 in which an output from the front microphone unit is subtracted by an output from the rear microphone unit. Under a condition in which a contact 2 is selected with the switch, the polarization voltage applied to the other microphone unit becomes higher than −60V and lower than 0V (for example −30V). Here, the microphone unit has hypercardioid characteristic as shown in “2” of FIG. 17. Under a condition in which a contact 3 is selected with the switch, polarization voltage of 0V (no polarization voltage) is applied to the other microphone unit. Here, the microphone unit has cardioid characteristic as shown in “3” of FIG. 17 in which only the front microphone unit performs the output. Under a condition in which a contact 4 is selected with the switch, the polarization voltage applied to the other microphone unit becomes higher than 0V and lower than 60V (for example 30V). Here, the microphone unit has wide cardioid characteristic as shown in “4” of FIG. 17. Under a condition in which a contact 5 is selected with the switch, the polarization voltage applied to the other microphone unit is +60V. Here, the microphone unit has omnidirectional characteristic as shown in “5” of FIG. 17 in which the output of the rear microphone unit is added to the output of the front microphone unit.
The directionality of the above described conventional variable directional microphone is variable by connecting two microphone units back-to-back and by making the polarization voltage applied to one of the microphone units variable or, as described above, by making the output level of each of the microphone units variable. However, this method of achieving variable directionality requires a complex circuit structure.
The directionalities the above described variable directional microphone can generally have are cardioid, bidirectional, and omnidirectional. An intermediate of the directionalities can be obtained through further providing alternatives for the mixing ratio of the outputs from the pair of microphone units or the level of the applied polarization voltages. However, this requires even more complex circuit structure.
With the exemplary circuit of FIG. 16, the directionality can be switched between several different levels. Here, the circuit requires the power supplies capable of generating voltages at different levels and the voltage must be selectable. Thus, the circuit structure is complex.
Directionality of handheld microphones widely used on stages and the like is cardioid or hypercardioid. A microphone having which directionality is to be used is chosen according to the sound the user prefers, in terms of preventing acoustic feedback, or the like. The variable directional microphone unit described above may be used but incorporating the switching circuit having such a complex structure as described above in a handheld microphone is difficult.
Therefore, a microphone is called for that has a simple circuit structure and enables the user to arbitrarily select the directionality from cardioid, hypercardioid, and supercardioid.